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The shape of the macroscopic second-order susceptibility tensor of molecules bound to a surface depends upon both the molecular tensor and the distribution of orientations. The details of these dependencies are discussed in general and in the special case of two photon electronic resonance enhancement. Using the results of an orientation averaging procedure, experiments, based upon variation of the angle of incidence, the input polarization, the output polarization, or the frequency, can be devised to determine the shape of a molecular tensor or the orientation distribution. Specific examples are given for interfacial phenol and cinnamic acid.

Third harmonic generation (THG) has been performed in reflection from thin polycrystalline films of chloro-gallium and fluoro-aluminum phthalocyanines. By comparison to THG from the 100 surface of silicon, the values of χ(3) are estimated to be 2.5×10−11 and 5×10−11 esu, respectively. The patterns of the molecular and crystal tensors are discussed for the case of multiple resonance enhancements.

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